The present invention relates generally to hydraulic lash adjusters, and more particularly to a hydraulic lash adjuster (HLA) of the type in which a check valve provides at least some of the leakdown flow from the pressure chamber to the reservoir.
Hydraulic lash adjusters (also sometimes referred to as "lifters") for internal combustion engines have been in use for many years to eliminate clearance, or lash, between engine valve train components under varying operating conditions, in order to maintain efficiency and to reduce noise and wear in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam through hydraulic fluid, trapped in a pressure chamber under a plunger. During each operation of the cam, as the length of the valve actuating components varies as a result of temperature changes and wear, small quantities of hydraulic fluid are permitted to enter the pressure chamber, or escape therefrom, thus effecting an adjustment in the position of the plunger, and consequently adjusting the effective total length of the valve train.
The cam operating cycle comprises two distinct events: (1) operation on the base circle and (2) valve actuation. The base circle event is characterized by a constant radius between the cam center of rotation and the cam follower, and during this event, no cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam center of rotation and the cam follower, which effectively transmits cam energy to open and close an engine valve. During the valve actuation event, a portion of the load resulting from the valve spring, the inertia of valve train components, and cylinder pressure are transmitted through the valve train and through the lash adjuster. The load increases the pressure of the hydraulic fluid within the lash adjuster pressure chamber, in proportion to the plunger area, and in typical hydraulic lash adjusters currently in commercial production, fluid escapes the pressure chamber between the plunger and the wall of the lash adjuster body. Such a device is referred to as a "conventional leakdown" lash adjuster.
As the fluid escapes, the volume of the pressure chamber is decreased and the plunger moves down, shortening the effective length of the lash adjuster. During the base circle event, the lash adjuster plunger spring moves the plunger up within the body such that no clearance or lash exists between valve actuation components. As this occurs, hydraulic fluid is drawn into the pressure chamber through the plunger check valve in response to the increasing volume of the pressure chamber as the plunger moves up. If the effective length of the valve train shortens during the valve actuation cycle, positive lash is created and the lash adjuster extends, moving the plunger to a higher position at the end of the cycle than at the beginning. Conversely, if the effective length of the valve train increases during the valve actuation cycle, negative lash is created and the lash adjuster contracts, moving the plunger to a lower position at the end of the cycle than at the beginning. The latter condition typically occurs when valve train components lengthen in response to increasing temperature.
As noted previously, commercial lash adjusters of the conventional leakdown type have controlled the escape of fluid from the high pressure chamber (or "leakdown") solely by the fit of the plunger within the body, thus necessitating close clearances therebetween, and selective fitting of the plunger to the body. In an effort to overcome the expense of such selective fitting of the plunger within the body, as well as other associated disadvantages of the prior art, the assignee of the present invention has developed a lash adjuster in which the leakdown flow is past the plunger check valve. This improved lash adjuster is illustrated and described in U.S. Pat. No. 5,622,147 for a "HYDRAULIC LASH ADJUSTER", assigned to the assignee of the present invention and incorporated herein by reference.
Lash adjusters of the type disclosed in the above-identified patent are referred to as having "sealed leakdown" systems. In connection with the development of the sealed leakdown type lash adjusters, it has been determined that it is necessary to assemble such lash adjusters into the engine without fluid (i.e., assemble the lash adjuster "dry"), and after the engine begins to run, oil is pumped into the lash adjuster in a manner well known to those skilled in the art. If such lash adjusters were already filled with hydraulic fluid, prior to assembly into the engine, the assembly process would be nearly impossible. In a sealed leakdown lash adjuster, the check valve constitutes the only fluid passage between the high pressure chamber and the reservoir so that a compressive load imposed on the plunger results in fluid being displaced through the check valve. During the assembly process, at normal assembly speeds, the hydrodynamic forces generated by the flow of fluid through the check valve are sufficient to close the check valve before the lash adjuster is shortened enough to permit assembly.
In response to the above-described problems, those skilled in the art have developed an HLA of the "normally biased open" type, i.e., the check valve comprises a check ball (also referred to as a "freeball" check valve), which is normally biased toward an open position. For example, in co-pending application U.S. Ser. No. 792,809, filed Jan. 30, 1997, now U.S. Pat. No. 5,758,613, for a "HYDRAULIC LASH ADJUSTER AND BIASED NORMALLY OPEN CHECK VALVE SYSTEM THEREFORE", assigned to the assignee of the present invention, and incorporated herein by reference, the check ball is biased by a compression spring toward the open position. Although the structure of the incorporated application functions in a satisfactory manner, the addition of the biasing spring adds to the overall expense of the HLA, and does add some complication to the assembly process.